Electromigration reliability is a serious concern for interconnects. Electromigration is an atomic-scale phenomenon in which the moving electrons that constitute electric-current collide with the metal-atoms of an interconnect line and push them in the direction of the current flow. Given enough collisions over time, a significant migration of the metal atoms will occur. This transport of material can lead to formation of voids and consequent increase in resistance, and ultimately lead to the electrical failure of the interconnect by breaking the line. Electromigration defects occur principally at interfaces between metal lines and overlying dielectric layers. In damascene interconnects, this is frequently the interface between copper and an etch stop/diffusion barrier layer of silicon nitride, silicon carbide or other dielectric capping layer.
As integrated circuit technology reduces to the 45 nm node and below, interconnect dimensions are being reduced without a corresponding reduction in the drive current. This puts a great deal of pressure on device designers to address the increased current density requirements. The problem is exacerbated because the critical void size for interconnect failures scales with the dimensions and further reduces reliability. One solution to improve electromigration reliability is to cap the top copper surface with a metal layer that has a greater resistance to electromigration than the copper. The cap layer can impede electromigration in two ways. First, it may adhere better to the copper than the dielectric barrier. Second, if the sheet resistance of the cap layer is low enough then it can provide a shunting layer near the contact, thereby mitigating the effect of any voids that form in the copper interconnect.
A metal cap layer may also supplement the barrier properties of the overlying etch stop layer deposited at the interface with the dielectric layer of the next higher metallization layer. In particular, depending on the composition and deposition technique used to form the metal capping layer, such layer may afford some independent protection against copper diffusion from metal lines to the dielectric in an overlying layer.
It would be desirable to have improved processes for depositing metal cap layers.